1. Field of the Invention
The present invention relates to a lubricating oil composition for automatic transmissions, and, more precisely, to that having an excellent ability to prevent the brake lining of the clutch of automatic transmissions from shuddering, and capable of maintaining the shudder-preventing ability for a long period of time.
2. Description of the Related Art
Automatic transmissions (AT), especially those equipped with a continuous slip mechanism are required to have the ability to prevent shudders that may occur in the clutch of AT. Cars using a conventional automatic transmission fluid (ATF) greatly shudder. Various attempts have been made to prevent cars from shuddering. For example, Japanese Patent Application Laid-Open No. 7-305082 discloses a lubricating oil composition for automatic transmissions, which comprises a base oil and additives of (1) zinc dithiophosphate and/or basic zinc dithiophosphate, (2) at least one compound selected from the group consisting of phosphates, acid phosphates and phosphites and (3) an organic friction regulator having a specific bond and/or a functional group as introduced into the molecule. This composition is effective in preventing initial shudders but is defective in that, while it is used for a long period of time, zinc dithiophosphate therein adheres to the surface of a wet clutch to thereby greatly shorten the shudder-preventing life of the composition, resulting in that the composition could no more maintain its original properties. Japanese Patent Application Laid-Open No. 63-254196 discloses the addition of a friction regulator of any of phosphates, fatty acid esters and fatty acid amides to ATF, which, however, is not satisfactory for preventing initial shudders.
Given the situation, the present invention is to provide a lubricating oil composition for automatic transmissions which has an excellent ability to prevent the brake lining of the clutch of automatic transmissions from shuddering, and can maintain the shudder-preventing ability for a long period of time.